FIG. 1 depicts a conventional exhaust system A for an outboard marine engine. Dry exhaust gas is conveyed from an internal combustion engine via a vertical exhaust pipe B to a lower gearcase cavity C, wherein the exhaust gas is allowed to expand. When the internal combustion engine is operated at above-idle speeds, most or all of the exhaust gas is discharged via an underwater outlet D, which typically is formed through the gearcase and an associated propeller assembly. When the internal combustion engine is operated at idle speed, the pressure associated with the body of water in which the propeller assembly is situated typically prevents a significant flow of the exhaust gas through the underwater outlet D. Most or all of the exhaust gas tends to take a path of least resistance to atmosphere, which is through an idle relief muffler E and then through an idle relief outlet F. The idle relief outlet F is located above the body of water in which the outboard marine engine is situated.
The following U.S. Patents disclose additional state of the art. These patents are incorporated herein by reference, in entirety:
U.S. Pat. No. 9,051,041 discloses a marine propulsion system for propelling a marine vessel in water. The system comprises an outboard motor that is coupled to a marine vessel. The system comprises an exhaust gas relief outlet that is located above the water when the outboard motor is operated at idle speed. A conduit conveys exhaust gas from the exhaust gas relief outlet to a discharge outlet located on the marine vessel.
U.S. Pat. No. 8,876,566 discloses a marine drive and marine exhaust pipe that include a main exhaust flow chamber and an auxiliary idle relief chamber. The auxiliary idle relief chamber vents exhaust above the surface of the body of water in which the vessel is operating.
U.S. Pat. No. 4,952,182 discloses an exhaust relief system for an outboard motor that includes an exhaust chamber into which exhaust is discharged from the engine. A first passage in communication with the exhaust chamber provides contraction of the exhaust as the exhaust passes rearwardly from which the exhaust is discharged into an expansion chamber which substantially surrounds the exhaust chamber. From the expansion chamber, the exhaust is routed through and contracted into a second passage in communication with the expansion chamber, after which it is discharged to atmosphere. The tortuous path provided by the exhaust relief system, along with the repeated expansion and contraction of the exhaust as it flows to atmosphere, provides a muffling effect at idle operation.
U.S. Pat. No. 4,668,199 discloses an exhaust system for an outboard motor that includes a main exhaust passageway extending through a partially water filled chamber in the drive shaft housing. An inlet idle relief passage connects the top of the chamber with the main exhaust passageway and an outlet passage connects the top of the chamber with the atmosphere.
U.S. Pat. No. 3,967,446 discloses a tuned exhaust gas relief system for marine propulsion systems, for example an outboard motor, that includes a lower drive shaft housing coupled to a two stroke engine by a pair of intermediate stacked exhaust extension plates. The housing directs the exhaust gas downwardly to a through-the-hub exhaust propeller for exit there through. With the unit in reverse or idling, exhaust gases are trapped within the housing. A pair of tuned exhaust relief passageways may be formed by cavities in the mating faces of the two extension plates with a pair of inlet openings in the lower wall of the bottom plate. A baffle member may overlie the inlet openings. The passageways define constant cross-sectional area channels which terminate in exhaust openings in the rear wall of the drive shaft housing.